It can be done on-set, as I believe most of the Were-Rabbit scenes were done -- but it's very time consuming. I think you can see a very fleeting glimpse of fog on set in this clip:https://youtu.be/DLVSEVQlXPA?t=1m21s

First, you'd need a fog machine. Just a cheap one from the Halloween store will do. Then, you have to be sure the room where the set is kept is completely closed off from the rest of the house; so close off all heating/cooling vents into that room and close all doors.

Once you're sure there wont be any stray airflow, then bring in a medium sized fan -- the reason being: you have to fog up the whole room, but the fog from the machine will come out in clouds. This is terrible for animation, because it moves in big plumes and the end result would look like time-lapse -- the fan is there so you can "grind" all those clouds into a fine haze. Haze won't move on camera and is perfect for an animation environment.

So, the process goes like this: Fill the room with fog, turn on the fan just long enough to particulate everything, then turn it back off. After that, continue your animation while monitoring the fog level in the room -- re-fog as needed (a little bit every 10 min. seems to work, but it may be different in your case, dependent on room size.)

As you can see, this is a tedious process -- and the inherent problem is that the fog levels will change on camera, even if you don't see it with the naked eye. The great thing about this method, however, is that after some practice, the result is really cool. I've personally used this method on several projects and it works quite well.

If, on the other hand, you'd like to do it in post, you'll need a decent compositing software and put some time in rotoscoping around the puppet to make the fog seem to pass behind it. Or shoot on a green screen and have the freedom to put fog wherever you like.

Thankyou for your detailed description, there’s so many little hidden tricks in the stopmotion world, thanks for that, I looks quite real and organic in the Wallace and Gromit film , it must be a lot of trial and error , thanks again Santino

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